This disclosure relates generally to a switching power supply and more particularly to a switching power converter detecting dynamic loads via a secondary-side detection device.
Many electronic devices, including smart phones, tablets, and portable computers, employ power supplies providing controlled and regulated power output over wide operating conditions. These power supplies often include a power stage for delivering electrical power from a power source to a load across a transformer. A switch in the power stage electrically couples or decouples the load to the power source, and a switch controller coupled to the switch controls on-time and off-time of the switch. Regulation of the power output can be accomplished by, among other things, measuring the output current or output voltage and feeding that back to the primary-side switch controller. In order to improve cost performance and reduce size, many commercially available isolated power supplies employ primary-only feedback and control. By sensing primary side signals, the secondary output and load condition can be detected and thus be controlled and regulated.
For convenience, end users often leave the power supply connected to the AC mains at times when no load is connected to the power supply output. To maintain a regulated output voltage even in no-load conditions, the controller may change its regulation mode under low load or no-load conditions. Under no-load conditions, the rate of the pulses that turn on or turn off the power switch of the switching power converter is decreased significantly in order to maintain output voltage regulation, resulting in long periods of time between ON and OFF cycles of the switching power converter. This presents a significant challenge to primary-side sensing control schemes that rely on the ON and OFF cycles of the power switch to obtain a feedback signal. During the periods between ON cycles of the switch, the status of the output voltage is unknown by the controller as no feedback signal is generated. If the electronic device is reconnected to the power supply, representing a dynamic load change, during one of the long OFF cycles of the switch, the primary-side controller does not receive feedback about the change in the secondary side output voltage until the next ON cycle of the switch. In the interim, the output voltage may therefore drop significantly, exceeding the allowable voltage drop specified by the regulation specifications of the switching power converter or the electronic device.